1. Field of the Invention
The present invention relates to a discharge lamp such as a mercury lamp, a metal halide lamp, or a high pressure sodium lamp, a discharge lamp lighting apparatus, and an illumination apparatus using the same.
More particularly, the present invention relates to a discharge lamp in which a nitride layer is formed on the surface of the tube wall of the envelope tube of a discharge tube assembly to prevent a reaction between a discharge medium sealed in the envelope tube and a tube wall material, thereby improving the performance.
2. Description of the Prior Art
In general, a material for the discharge tube assembly of a high pressure metal vapor discharge lamp is selected in consideration of transparency, heat resistance, chemical resistance, workability, and the like. A discharge tube assembly consisting of quartz is used for a high pressure mercury lamp or a metal halide lamp, whereas a discharge tube assembly consisting of a transparent ceramic material, e.g., an alumina (Al.sub.2 O.sub.3) ceramic material is used for a high pressure sodium lamp.
Even if the above discharge tube assembly materials are used for such discharge lamps, the problem of a decrease in luminous flux after a long-term use still remains unsolved.
There are various causes of a decrease in luminous flux. One of the causes is a reaction between a discharge medium sealed in a discharge tube assembly and a discharge tube assembly material.
For example, in a metal halide lamp, when a metal halide or a discharge metal dissociated therefrom, which is sealed in a discharge tube assembly consisting of quartz, reacts with the quartz, discoloration of the quartz occurs. For this reason, the light transmission characteristics deteriorate. A decrease in luminous flux is also caused by a reduction in the amount of discharge metal. As a result, the luminous flux maintaining rate decreases.
In a high pressure sodium lamp, a reaction product is produced by a reaction between sodium or sodium ions sealed in a discharge tube assembly consisting of an alumina ceramic material and the discharge tube assembly. As a result, so-called sodium loss occurs, and an increase in discharge voltage or a decrease in luminous flux occurs.
In a medium pressure mercury lamp, mercury sealed in a discharge tube assembly consisting of quartz is injected into the quartz. As a result, the discharge tube assembly is blackened.
In order to eliminate such drawbacks, Jpn. Pat. Appln. KOKOKU Publication No. 57-44208 discloses a technique of coating a silicon nitride (Si.sub.3 N.sub.4) film on the inner surface of a discharge tube assembly. According to this technique, when the silicon nitride film described in the above official gazette is formed on the inner surface of the discharge tube assembly, the film moderate a reaction between the discharge metal and the discharge tube assembly, and prevents removal of the discharge medium, thereby preventing a decrease in luminous flux and maintaining a high luminous flux.
In addition, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-262358, an aluminum nitride coating is formed on the inner surface of an alumina discharge tube assembly to decrease the temperature of a central portion of the discharge tube assembly, thereby reducing sodium loss.
In either of the above conventional techniques, however, since the silicon nitride or aluminum nitride film, which is different from the material of the tube wall of the envelope tube of the discharge tube assembly, is formed on the inner surface of the envelope tube, the discharge tube assembly material is different from the film material in thermal expansion coefficient. For this reason, the silicon nitride or aluminum nitride film may undergo cracking, peeling, removal, or the like.
In the latter official gazette, in order to solve this problem, the thickness of the aluminum nitride film is set to be 5 .mu.m or less. Even if, however, the film thickness is set to be 5 .mu.m or less, cracking or the like may take place. That is, it is difficult to form a sufficiently effective film. No such a film has been put into practice.
It is an object of the present invention to provide a discharge lamp in which a chemically and physically stable nitride layer is formed on the surface of the envelope tube of a discharge tube assembly to prevent a reaction between a discharge medium and the material of the envelope tube of the discharge tube assembly and removal of the discharge medium, thereby maintaining a high luminous flux maintaining rate and preventing cracking, peeling, removal, and the like, and an illumination apparatus using this discharge lamp.